This invention relates to semiconductor relay circuits and, more particularly, to an isolated semiconductor relay circuit. Isolation is provided by an arrangement of a circuit for converting a light signal into an electric signal by means of an array of photovoltaic diodes optically coupled to light emitting means, such as a light emitting diode, that converts an input signal into the light signal, and for driving an output field-effect transistor (which shall be simply referred to as "FET" hereinafter) by means of the electric signal into which the light signal is converted.
1. Field of the Invention
The semiconductor relay circuit of the kind referred to can be effectively utilized in, for example, a system of transmitting and controlling electric signals in metering and measuring devices and equipment.
2. Description of the Related Art
For the semiconductor circuit of the kind referred to, there has been suggested in, for example, U.S. Pat. No. 4,390,790 of E.T. Rodriguez a circuit in which a MOSFET is series connected in to a photovoltaic diode array optically coupled to a light emitting diode. A normally ON type junction FET is connected between the gate and source of this MOSFET, and a further photovoltaic diode array is connected through a resistor across the gate and source of the junction FET. In this U.S. patent, the MOSFET is prevented from being kept in intermediate state by arranging the normally ON type junction FET to be driven by another diode array, so as to achieve a high speed operation to some extent. Since the circuit arrangement combines the junction FET with another diode array, on the other hand, a problem arises that the diode arrays generally require a larger chip size. This deteriorates optical transmission efficiency and lowers the operation speed. When the output FET turns completely ON and the voltage across the drain and source becomes substantially zero, the current from the photovoltaic diode array is likely to flow across the drain and source of the output FET, so that the high speed operation can be attained only on such rare occasions that the output FET has a considerably high ON resistance and its drain voltage has such a higher level than the gate voltage as, normally, about 5V or more, which has been troublesome in that the normal operation range is thereby restricted.
In Japanese Utility Model Application Laid-Open Publication No. 64-33228 of Y. Nozaki et al, discloses a measure for speeding up the operation of MOSFET by connecting a phototransistor through a controlled-part power source across the drain and gate of the MOSFET and supplying a current from such power source through the phototransistor conducting to the gate of the MOSFET. According to this measure, however, a higher speed operation may be attained upon the turning-on but the speeding-up of the turning-off operation is still deficient due to the lack of any discharging control circuit across the gate and source of MOSFET. This leaves the problem unsolved in that the reverse flowing of the photocurrent between the gate and drain of MOSFET cannot be restrained and the normal operation range is thereby limited, for the same reason as in the foregoing known arrangement.
According to an earlier invention by the same inventors as in the present case, as disclosed in U.S. Pat. No. 4,801,822, there is provided a semiconductor circuit, in which a MOSFET is connected to a photovoltaic diode array optically coupled to a light emitting element. A depletion-mode driving FET is connected between the gate and source of this MOSFET, and a parallel circuit of a resistor and a constant voltage conduction element is connected across the gate and source of the driving FET, so that the constant voltage conduction element will be made conductive upon application of a voltage higher than that at a threshold level of the depletion mode driving FET, and an output FET will be charged across the gate and source and discharged across the drain and gate of an output FET by means of a conduction current upon the conduction of the element. While this circuit arrangement may attain a sufficiently satisfactory higher speed turning-off, it is still deficient in attaining the higher speed operation upon the turning-on.